268 PALEONTOLOGY OF NEW YORK. 



Shells plano-convex or concavo-convex. Pedicle-valve with acute incurved 

 beak, perforated at its extremity, the foramen being continuous with a partially 

 closed delthyrium; medially ridged, and with abruptly sloping sides. On the 

 interior the teeth are large, thick at their extremities and adherent to the 

 lateral walls of the shell. Between them is a deep pedicle-cavity, in the bottom 

 of which lie the elongate scars of the adductor muscles, and about their ante- 

 rior portion the small, flabellate diductors. 



The brachial valve is very shallow, rendered concave exteriorly by a median 

 sinus which does not make itself apparent on the interior. Beak small, apex 

 not incurved. Dental sockets broad, bounded interiorly by the high walls of 

 the hinge-plate. This plate is divided medially by a deep furrow extending to 

 the apex, and therefore consists of two processes which are elevated, thickened 

 and rest on the bottom of the valve. From the anterior face of these arise 

 the crura which converge for a short distance, and 

 expand to form two broad acute jugal processes. 

 From here the lateral branches of the brachidium 

 curve outward, gradually turning from a vertical to 

 a horizontal position, broaden rapidly and unite to no. iso. 



Centronella glans/agea. Hall. 



form an anterior triangular plate which bears a ^ prepaiatioa showing the brachidium. 

 median ridge, where the two lateral branches are 



conjoined. The whole of the anterior portion of the brachidium is inclined 

 gently upward toward the cavity of the opposite valve. 



The muscular impressions occupy an elongate area below the hinge-plate, 

 and are divided by a median ridge, but are only obscurely divisible into their 

 elementary scars. The lateral portions of this valve frequently bear a series 

 of vascular sinuses in the pallial region. 



Surface smooth or with concentric lines crowded near the margins of the 

 valves. Shell-substance punctate. 



Type, Rhynchonella glans-fagea, Hall. Upper Helderberg group. 



Observations. The type of external form and internal structure exemplified 

 by this species is probably also represented by the C. alveata, Hall (= C. Hecate, 



